This invention relates generally to the formation of particle composite products, and more particularly to a method of forming particle composite products from ligno-cellulosic material. As used herein, the term ligno-cellulosic material is intended to include logs, lumber, wood particles, wood chips, wood flakes, wood wafers, wood fibers, wood veneer and other wood products and parts thereof, as well as other lignin and cellulose containing matter, such as woody plants, foliage, roots, shells, pot, nuts, husks, fibers, straw, vine, grass, bamboo, and reeds.
The ever increasing demand for wood and the rising costs associated therewith have driven the wood industry to develop products made of alternative materials, such as scrap or production based materials. One particular wood-based product is composite particle products that are formed from relatively small ligno-cellulosic material. Products based on these composite particle products that have been bonded together to form a comparatively homogeneous sheet by means of an adhesive have garnered considerably popularity in the building industry. The term "composite particle product" is intended to include those products made from the bonding together of relatively small ligno-cellulosic material, such as material having a size smaller than veneer sheets and preferably smaller than two inches, and is intended to include particleboard, flakeboard, oriented strand board (OSB), plywood, hardboard, softboard, chipboard and the like.
The manufacture of composite particle products are known in the art. Generally, the composite products are formed from ligno-cellulosic material fragments or pieces, such as wood chips, flakes, strands and fibers, that are bonded together via a suitable heat-hardenable thermosetting adhesive under selected pressure and temperature conditions to form the product. The ligno-cellulosic material used is generally manufactured from the residue of milling operations, such as planer shavings, sawdust, plywood trimmings, and the like. Milling residues may be further reduced to an appropriate or desired size in a hammermill operation.
If the size of the ligno-cellulosic material is such that it needs to be reduced further, it is generally conditioned in water prior to reduction in a flaking or chipping machine. The reduced material is known as furnish. The furnish is then dried to a substantially uniform moisture, usually ranging from between 3% and 6%, in known drying apparatus. The dried furnish is then screened to segregate selected particle sizes. For example, fine particles are used to form the exposed surface of the composite products to produce a smooth surface, or are used as fuel for the composite forming process.
The furnish is then introduced to a blender and the heat-hardenable thermosetting adhesive, such as urea-formaldehyde or phenol-formaldehyde, is added. These thermosetting resins typically require the application of heat and pressure to cure them. The furnish and adhesive is then deposited on a belt conveyor system in a loose mat form and is transported to a heat press reactor station, which heats and presses the mat to cure (harden) the adhesive resin. The heat press station is typically maintained at relatively low temperatures to avoid carbonizing the mat during curing. During this curing stage, the adhesive is thermally fused with the ligno-cellulosic material. The resultant formed composite particle product is then cooled and transported to a downstream finishing station where the product is embossed, finished and cut to any appropriate size. This product formation technique is well known in the art as disclosed in U.S. Pat. No. 4,933,125 of Reiniger, U.S. Pat. No. 4,517,147 of Taylor et al., U.S. Pat. No. 5,154,968 of DePetris et al., and U.S. Pat. No. 4,726,881 of Schultz. The predominant product produced from the foregoing system is composite sheets that are used as underflooring or in other building applications.
A drawback of the foregoing composite product forming systems is that the resultant sheet product is sensitive to moisture. This sensitivity can be attributed on the one hand to the type of adhesive used, and on the other hand to the ligno-cellulosic material. Specifically, the product absorbs moisture, which causes the ligno-cellulosic material to expand, overcoming the binding forces of the hardened adhesive. This results in splitting and cracking of the composite product.
The conventional systems further employ either aminoresins, such as urea-formaldehyde, or phenolic resins, such as phenol-formaldehyde as thermosetting adhesives. Aminoresins are polymeric products of the reaction of an aldehyde with compounds as thermosetting adhesives containing amino groups, particularly urea and melamine. In nearly all aminoresins, the aldehyde component is formaldehyde and by far the dominant aminoresin is urea-formaldehyde. A major disadvantage of aminoresins are that they are not completely water-resistant, and consequently are known to delaminate during use. Another drawback is that they are known to leach formaldehyde during their slow water hydrolysis.
Phenolic resins are polymeric products of the reaction between an aldehyde, a compound containing a phenolic hydroxyl group. The phenolic component is oftentimes phenol, but may also be cresol, resorcinol, or catechal. Formaldehyde is the most common aldehyde component although others such as furfural are occasionally used. The most common phenolic-type resin is phenol-formaldehyde resin. Phenol and the other phenolic substances are considerably more expensive than urea, but maintain their seam lines in the presence of moisture.
A drawback of the foregoing resins are that they are petroleum derived compounds, and thus are expensive and subject to the seasonal fluctuations in petroleum prices. Cost-wise these resins comprise a substantial portion of the overall system cost. Hence, it would be advantageous to employ a less expensive, non-petrochemical phenolic adhesive material. Another drawback of use of these resins is that formaldehyde is toxic, and thus concerns regarding outgassing or leakage of formaldehyde over time is of the utmost concern.
In an effort to overcome the foregoing drawbacks, the art began to employ preprocessed lignin in combination with one of the foregoing resins to help reduce the cost of the overall process. Lignin, as it occurs in all woody plants, is a three-dimensional macromolecule consisting of rather hydrophobic phenylpropane units. Commercial lignin is generally available as a by-product of the pulping process during the manufacture of paper. For example, lignosulfonate is obtained from the sulfite pulping process in which wood is cooked with sulfite in a liquor bath at elevated temperatures and at a selected pH level. In this process, the lignin is rendered water-soluble. This lignin component can then later be removed from the spent sulfite liquor.
Conventional techniques for modifying or preprocessing lignin into a water-soluble product for use as a binder in various wood processes exist. One prior art technique for solubilizing lignin is the methylolation of lignin, e.g., sulfite lignin. For example, as described in U.S. Pat. No. 4,332,589 of Lin, lignin is methylolated by treating lignin with formaldehyde under alkaline conditions at a temperature between about 60.degree. C. and about 90.degree. C. The resultant lignin is then acidified to a pH below 7 and heated to a higher elevated temperature. This technique for solubilizing sulfite lignin is further set forth in U.S. Pat. No. 4,244,840 of Edler and in U.S. Pat. No. 5,075,402 of Schmitt et al.
The methylolation of kraft lignin was also described in U.S. Pat. No. 5,202,403 of Doering. Doering describes the formation of a lignin modified phenol-formaldehyde adhesive compound formed by reacting phenol with formaldehyde to form a precursor resin in the presence of an alkaline material, such as sodium hydroxide. The resin is then heated to further polymerize the reactants, and then a lignin source, such as kraft lignin, is added in a controlled manner. The ligno-resin formed from this process can then be used in wood preparation processes as an adhesive.
Another prior art technique for preprocessing lignin includes the demethylation of lignin. This treatment increases the amount of aromatic hydroxilic groups in lignin, rendering the lignin water-soluble. This technique is described in U.S. Pat. No. 5,177,169 of Schroeder.
Simonson et al., U.S. Pat. No. 4,892,618, describes a method of forming adhesive bonded wood products using lignin containing compounds. In alkaline solution Kraft lignin is water-soluble and is used to impregnate the material. The solubilized ligni is then converted back to its water-insoluble form while resident within the material upon reaction with an aluminum salt, such as sulfate or chloride. According to the described technique, kraft lignin is reacted with an alkaline material (NaOH), and is then precipitated and purified. Wood is then impregnated with the solubilized lignin compound. A conventional adhesive, such as melamine-urea-formaldehyde adhesive, was used to adhere the wood pieces.
A significant disadvantage of the foregoing techniques is that they require time-consuming and cost intensive preprocessing and transformation of lignin into a water-soluble form for use in adhesive compounds. This processing adds substantially to production costs since lignin is converted from a naturally water-insoluble form into a water-soluble form.
Due to the foregoing and other shortcomings of the foregoing prior art technologies, an object of this invention is to provide a novel system and method for employing lignin as an adhesive to bond together ligno-cellulosic material.
Another object of the invention is to provide a cost effective system and method for forming composite particle products employing lignin as a binder.
Still another object of the invention is to provide a method for forming a composite product by impregnating wood with lignin.
Yet another object of the invention is to provide a relatively inexpensive system and method for producing composite particle products.
Other general and more specific objects of the invention will in part be obvious and will in part appear from the drawings and description which follow.